vpython: Add primary execution package.

vpython/system_posix.go

+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// +build darwin linux freebsd netbsd openbsd android
+
+package vpython
+
+import (
+        "os"
+        "syscall"
+)
+
+// forwardedSignals is the list of signals that should be forwarded to the
+// Python subproess.
+var forwardedSignals = []os.Signal{
+        syscall.SIGHUP,
+        syscall.SIGINT,
+        syscall.SIGQUIT,
+        syscall.SIGUSR1,
+        syscall.SIGUSR2,
+}